Fans comprising a motor and air blades are well known in the art. They are typically used to cool a hot object (e.g. a processor or a hard disk) by forcing an air stream over the hot object, in a manner known as “forced cooling”.
Different kinds of motors are used in a fan, ranging from a single coil motor to a three coil motor. Both DC motors (with brushes) and brushless DC motors (BLDC) are typically used.
As BLDC motors do not have brushes, they typically create less noise and less wear than their DC counterparts, but the currents of such a motor need to be applied by a driver circuit according to a suitable “commutation scheme”, meaning that the amplitude and/or direction of the currents flowing through each of the coils needs to be adjusted depending on the angular position of the rotor to make the motor move as desired (e.g. to start the motor, accelerate, decelerate, maintain constant speed, etc.). Despite the more complex driving scheme, BLDC motors are often chosen because they are less subject to wear.
There are three major categories of schemes for driving BLDC motors:
(1) The first and most complex driving scheme is known as “Field Oriented Control” (FOC), in which case an optimal vector is calculated in real time based on up-to-date information about the motor and its load. This scheme typically requires a fast digital signal processor (DSP) which is typically too expensive for fan applications.
(2) The second scheme (medium complexity) approximates or emulates FOC by applying sinusoidal currents, and is typically based on look-up tables and only partial information about the motor and its load. Two implementations typically fall in this category: (a) Sine-wave BLDC motor control, which uses motor information to achieve high efficiency, and (b) so called “micro-stepping”, where the sinusoidal waveform is approximated by a small number of steps. Micro-stepping can be used when efficiency is less critical. Both schemes can be generated by a relatively simple microcontroller with dedicated motor control hardware (such as PWM blocks).
(3) The third category (lowest complexity) uses “trapezoidal waveforms”.
A nice overview of these commutation methods is described in the paper “A COMPARISON STUDY OF THE COMMUTATION METHODS FOR THE THREE-PHASE PERMANENT MAGNET BRUSHLESS DC MOTOR” by Shiyoung Lee and Tom Lemley (7 pages).
It shall be clear that many different designs of motor drive circuits exist, and different trade-offs can be made, not only in terms of maximum power or maximum current, but also in terms of energy efficiency, torque ripple, acoustical noise, but also in terms of functionality, such as for example guaranteed start-up behaviour, or the ability to control the motor speed (or not), or the ability to detect motor stall (or not), etc. It is noted that for some fan applications it suffices that the motor runs at a substantially predefined constant speed, while other fan applications require speed control.
In many applications, the main consideration is energy or power efficiency. The aspect of power efficiency is not only important for battery powered devices (such as laptops) but also for reducing emission of combustion engines of vehicles. It is evident that no electrical energy should be wasted, hence many existing drive circuits are designed to drive the motor in the most energy-efficient manner. This usually also results in a reduced heat dissipation, requires lower currents, lower drive capabilities, and thus typically results in lower hardware and/or package costs.
Some motor driver circuits are designed to address specific problems. For example, EP16154767 (filed by the same applicant) describes a relatively sophisticated drive circuit for driving a fan at a variable speed according to specific requirements, where the design is based primarily on considerations of torque and acoustical noise.
The present invention is related to another specific problem, namely to provide a low cost driver circuit, especially designed for starting and/or driving a fan in the automotive temperature range from −40° C. to +85° C. or higher, and is particularly related to problems encountered at the bottom end of this temperature range.